This invention relates to package encapsulant compositions for electronic devices that protect the electronic component and its metallization from environmental corrosion and mechanical damage.
Microelectronic devices contain millions of electrical circuit components, mainly transistors assembled in integrated circuit (IC) chips, but also resistors, capacitors, and other components. The integrated circuit component may comprise a single bare chip, a single encapsulated chip, or an encapsulated package of single or multi-chips. These electronic components are interconnected to form the circuits, and eventually are connected to and supported on a carrier or substrate, such as a printed wire board.
The various materials used to manufacture the integrated circuits and their related interconnect materials are susceptible to environmental, moisture, and mechanical damage. Protection is provided by encapsulation of the electronic component within a polymeric material. Encapsulation can be performed by a transfer molding process in which the component is loaded into a mold cavity, constrained, and the polymeric encapsulant transferred from a reservoir into the cavity under pressure. Typically, the encapsulant is a thermosetting polymer, which then cross-links and cures to form the final assembly. Encapsulation also can be performed by dispensing an aliquot of polymeric encapsulant onto the component, such as a chip or integrated circuit supported on a substrate, and subsequently curing the composition.
For most commercial and industrial end uses, particularly those utilizing chip-on-board packages and multi-chip modules, the encapsulation is accomplished with polymeric thermosetting materials. The preferred thermosetting package encapsulation materials must have a viscosity and a thixotropic index that allows easy dispensability by syringe, sufficient adhesion to the components, low ionic content to avoid corrosion of the metallization, adequate mechanical strength, high thermal and moisture resistance at application temperatures, and matched coefficient of thermal expansion to the materials it contacts.
For single chip packaging involving high volume commodity products, a failed chip can be discarded without significant loss. However, it becomes expensive to discard multi-chip packages with only one failed chip, and the ability to rework the failed component would be a manufacturing advantage. Today, one of the primary thrusts within the semiconductor industry is to develop not only a package encapsulant that will meet all the requirements for protection of the component, but also a package encapsulant that will be reworkable, allowing for the failed component to be removed without destroying the substrate.
In order to achieve the required mechanical performance and reworkability, relatively high molecular weight thermoplastics would be the preferred compositions for package materials. These materials, however, have high viscosity or even solid film form, which are drawbacks to the manufacturing process. Therefore, there is a need for new encapsulant compositions that are easily dispensable to conform with automated manufacturing processes, and that are reworkable.
This invention is a curable encapsulant composition for electronic components that comprises one or more compounds containing one or more maleimide functionality, or one or more compounds containing one or more vinyl functionality, or a combination of compounds containing maleimide or vinyl functionality, a free-radical initiator and/or a photoinitiator, and optionally one or more fillers.
A compound containing one maleimide functionality will be referred to hereinafter as a mono-functional maleimide compound. A compound containing more than one maleimide functionality will be referred to hereinafter as a poly-functional maleimide compound. A compound containing one vinyl functionality will be referred to hereinafter as a mono-functional vinyl compound. A compound containing more than one vinyl functionality will be referred to hereinafter as a poly-functional vinyl compound. The functionality is defined herein to be a carbon to carbon double bond.
In another embodiment, this invention is also a cured encapsulant composition that results after the curing of the just described curable encapsulant composition.
In another embodiment, this invention is an electronic component electrically and mechanically connected to a substrate, encapsulated in a cured encapsulant composition, in which the cured encapsulant was prepared from a composition comprising one or more mono- or polyfunctional maleimide compounds, or one or more mono- or polyfunctional vinyl compounds other than maleimide compounds, or a combination of maleimide and vinyl compounds, a free radical curing agent and/or a photoinitiator, and optionally one or more fillers.
The maleimide and vinyl compounds used in the package encapsulant compositions of this invention are curable compounds, meaning that they are capable of polymerization, with or without crosslinking. As used in this specification, to cure will mean to polymerize, with or without crosslinking. Cross-linking, as is understood in the art, is the attachment of two polymer chains by bridges of an element, a molecular group, or a compound, and in general will take place upon heating. As cross-linking density is increased, the properties of a material can be changed from thermoplastic to thermosetting, which consequently increases polymeric strength, heat-and electrical resistance, and resistance to solvents and other chemicals.
It is possible to prepare polymers of a wide range of cross-link density, from tacky, elastomeric to tough glassy polymers, by the judicious choice and amount of mono- or polyfunctional compounds. The greater proportion of polyfunctional compounds reacted, the greater the cross-link density.
If thermoplastic properties are desired, the package encapsulants of this invention can be prepared from mono-functional compounds to limit the cross-link density. However, a minor amount of poly-functional compounds can be added to provide some cross-linking and strength to the composition, provided the amount of poly-functional compounds is limited to an amount that does not diminish the desired thermoplastic properties. Within these parameters, the strength and elasticity of individual package encapsulants can be tailored to a particular end-use application. The cross-link density can also be controlled to give a wide range of glass transition temperatures in the cured encapsulant to withstand subsequent processing and operation temperatures.
In those cases where it is necessary to rework the assembly, a thermoplastic composition should be chosen so that the electronic component can be pried off the substrate. Any residue package encapsulant can be heated until it softens and then be easily removed.
In the inventive package encapsulant compositions, the maleimide compounds and the vinyl compounds may be used independently, or in combination. The maleimide or vinyl compounds, or both, will be present in the curable package encapsulant compositions in an amount from 2 to 98 weight percent based on the organic components present (excluding any fillers).
The package encapsulant compositions will further comprise at least one free-radical initiator, which is defined to be a chemical species that decomposes to a molecular fragment having one or more unpaired electrons, highly reactive and usually short-lived, which is capable of initiating a chemical reaction by means of a chain mechanism. The free-radical initiator will be present in an amount of 0.1 to 10 percent, preferably 0.1 to 3.0 percent, by weight of the maleimide or vinyl compound, or a combination of both maleimide and vinyl compounds (excluding any filler). The free radical curing mechanism gives a fast cure and provides the composition with a long shelf life before cure. Preferred free-radical initiators include peroxides, such as butyl peroctoates and dicumyl peroxide, and azo compounds, such as 2,2xe2x80x2-azobis(2-methyl-propanenitrile) and 2,2xe2x80x2-azobis(2-methyl-butanenitrile).
Alternatively, the encapsulant compositions may contain a photoinitiator, such as is sold by Ciba Specialty Chemicals under the trademark Irgacure, in lieu of the free-radical initiator, and the curing process may then be initiated by UV radiation. The photoinitiator will be present in an amount of 0.1 to 10 percent, preferably 0.1 to 3.0 percent, by weight of the maleimide or vinyl compound, or a combination of both maleimide and vinyl compounds (excluding any filler). In some cases, both photoinitiation and free-radical initiation may be desirable. For example, the curing process can be started by UV irradiation, and in a later processing step, curing can be completed by the application of heat to accomplish a free-radical cure.
In general, these compositions will cure within a temperature range of 80xc2x0 to 180xc2x0 C., and curing will be effected within a length of time of 5 minutes to 4 hours. As will be understood, the time and temperature curing profile for each encapsulant composition will vary, and different compositions can be designed to provide the curing profile that will be suited to the particular industrial manufacturing process.
Ease of application, even when thermoplastic properties are desired for the package encapsulant, is achieved by using relatively low molecular weight reactive oligomers or pre-polymers and curing these in situ after application to the electronic assembly of component and substrate. Applying the materials in an uncured state gives high processibility, and the resultant cured thermoplastic encapsulant provides high mechanical performance.
For some packaging operations, inert inorganic fillers are used in the package encapsulant to adjust the coefficient of thermal expansion to more closely mirror that of the circuit interconnect, and to mechanically reinforce the interconnect. Examples of suitable thermally conductive fillers include silica, graphite, aluminum nitride, silicon carbide, boron nitride, diamond dust, and clays. The fillers will be present typically in an amount of 20-80 percent by weight of the total package encapsulant composition.
As used throughout this specification: the notation C(O) refers to a carbonyl group.
The maleimide compounds suitable for use in the package encapsulant compositions of this invention have a structure represented by the formula: [Mxe2x80x94Ar m]nxe2x80x94Q, or by the formula: [Mxe2x80x94Z m]nxe2x80x94Ar. For these specific formulae, when lower case xe2x80x9cnxe2x80x9d is the integer 1, the compound will be a mono-functional compound; and when lower case xe2x80x9cnxe2x80x9d is an integer 2 to 6, the compound will be a poly-functional compound.
Formula [Mxe2x80x94Ar m]nxe2x80x94Q represents those compounds in which: M is a maleimide moiety having the structure 
in which
R1 is H or C1 to C5 alkyl;
each Ar independently is an aromatic group selected from the aromatic groups having the structures (I) through (V): 
and Q is a linear or branched chain alkyl, alkyloxy, alkyl amine, alkyl sulfide, alkylene, alkyleneoxy, alkylene amine, alkylene sulfide, aryl, aryloxy, or aryl sulfide species having up to about 100 atoms in the chain, which may contain saturated or unsaturated cyclic or heterocyclic substituents pendant from the chain or as part of the backbone in the chain, and in which any heteroatom present may or may not be directly attached to Ar;
or Q is a urethane having the structure: 
xe2x80x83in which each R2 independently is an alkyl, aryl, or arylalkyl group having 1 to 18 carbon atoms; R3 is an alkyl or alkyloxy chain having up to 100 atoms in the chain, which chain may contain aryl substituents; X is O, S, N, or P; and n is 0 to 50;
or Q is an ester having the structure: 
xe2x80x83in which R3 is an alkyl or alkyloxy chain having up to 100 atoms in the chain, which chain may contain aryl substituents;
or Q is a siloxane having the structure: xe2x80x94(CR12)exe2x80x94[SiR4xe2x80x94O]fxe2x80x94SiR42xe2x80x94(CR12)gxe2x80x94 in which the R1 substituent independently for each position is H or an alkyl group having 1 to 5 carbon atoms and the R4 substituent independently for each position is an alkyl group having 1 to 5 carbon atoms or an aryl group, and e and g are independently 1 to 10 and f is 1 to 50; and
m is 0 or 1, and n is 1 to 6.
Preferably, Ar is structure (II), (III), (IV) or (V), and more preferably is structure (II).
Preferably, Q is a linear or branched chain alkyl, alkyloxy, alkylene, or alkyleneoxy species having up to about 100 atoms in the chain, as described with pendant saturated or unsaturated cyclic or heterocyclic substituents, or a siloxane as described, and more preferably is a linear or branched chain alkyl species or siloxane, as described.
Formula [Mxe2x80x94Z m]nxe2x80x94Ar represents those compounds in which M is a maleimide moiety having the structure 
in which
R1 is H or C1 to C5 alkyl;
Z is a linear or branched chain alkyl, alkyloxy, alkyl amine, alkyl sulfide, alkylene, alkyleneoxy, alkylene amine, alkylene sulfide, aryl, aryloxy, or aryl sulfide species having up to about 100 atoms in the chain, which may contain saturated or unsaturated cyclic or heterocyclic substituents pendant from the chain or as part of the backbone in the chain, and in which any heteroatom present may or may not be directly attached to K;
or Z is a urethane having the structure: 
xe2x80x83in which each R2 independently is an alkyl, aryl, or arylalkyl group having 1 to 18 carbon atoms; R3 is an alkyl or alkyloxy chain having up to 100 atoms in the chain, which chain may contain aryl substituents; X is O, S, N, or P; and n is 0 to 50;
or Z is an ester having the structure: 
xe2x80x83in which R3 is an alkyl or alkyloxy chain having up to 100 atoms in the chain, which chain may contain aryl substituents;
or Z is a siloxane having the structure: xe2x80x94(CR12)exe2x80x94[SiR42xe2x80x94O]fxe2x80x94SiR42xe2x80x94(CR12)gxe2x80x94 in which the R1 substituent independently for each position is H or an alkyl group having 1 to 5 carbon atoms and the R4 substituent independently for each position is an alkyl group having 1 to 5 carbon atoms or an aryl group, and e and g are independently 1 to 10 and f is 1 to 50;
Ar is an aromatic group selected from the aromatic groups having the structures (VI) through (XIII) (although only one bond may be shown to represent connection to the aromatic group K, this will be deemed to represent any number of additional bonds as described and defined by n): 
in which p is 1 to 100; 
in which p is 1 to 100; 
in which
R5, R6, and R7 are a linear or branched chain alkyl, alkyloxy, alkyl amine, alkyl sulfide, alkylene, alkyleneoxy, alkylene amine, alkylene sulfide, aryl, aryloxy, or aryl sulfide species having up to about 100 atoms in the chain, which may contain saturated or unsaturated cyclic or heterocyclic substituents pendant from the chain or as part of the backbone in the chain, and in which any heteroatom present may or may not be directly attached to the aromatic ring; or R5, R6, and R7 are a siloxane having the structure xe2x80x94(CR12)exe2x80x94[SiR42xe2x80x94O]fxe2x80x94SiR42xe2x80x94(CH3)gxe2x80x94 in which the R1 substituent is H or an alkyl group having 1 to 5 carbon atoms and the R4 substituent independently for each position is an alkyl group having 1 to 5 carbon atoms or an aryl group, and e is 1 to 10 and f is 0 to 50; 
and m is 0 or 1, and n is 1 to 6.
Preferably, Z is a linear or branched chain alkyl, alkyloxy, alkylene, or alkyleneoxy species having up to about 100 atoms in the chain, as described with pendant saturated or unsaturated cyclic or heterocyclic substituents, or a siloxane as described, and more preferably is a linear or branched chain alkyl species or siloxane, as described.
Preferably, Ar is structure (VIII), (X) or (XI), more preferably is structure (X) or (XI), and most preferably is structure (X).
The more preferred maleimide compounds, particularly for use as reworkable encapsulants, are N-butylphenyl maleimide and N-ethylphenyl maleimide.
The vinyl compounds (other than the maleimides) herein will have the structure: 
For these specific structures, when lower case xe2x80x9cnxe2x80x9d is the integer 1, the compound will be a mono-functional compound; and when lower case xe2x80x9cnxe2x80x9d is an integer 2 to 6, the compound will be a poly-functional compound.
In these structures, R1 and R2 are H or an alkyl having 1 to 5 carbon atoms, or together form a 5 to 9 membered ring with the carbons forming the vinyl group; B is C, S, N, O, C(O), Oxe2x80x94C(O), C(O)xe2x80x94O, C(O)NH or C(O)N(R8), in which R8 is C1 to C5 alkyl; m is 0 or 1; n is 1-6; and Ar, Q, and Z are as described above.
Preferably, B is O, C(O), Oxe2x80x94C(O), C(O)xe2x80x94O, C(O)NH or C(O)N(R8); more preferably B is O, C(O), Oxe2x80x94C(O), C(O)xe2x80x94O, or C(O)N(R8).
Depending on the nature of the substrate to which the package encapsulant is to be bonded, the encapsulant may also contain a coupling agent. A coupling agent as used herein is a chemical species containing a polymerizable functional group for reaction with the maleimide and other vinyl compound, and a functional group capable of condensing with metal hydroxides present on the surface of the substrate. Such coupling agents and the preferred amounts for use in compositions for particular substrates are known in the art. Suitable coupling agents are silanes, silicate esters, metal acrylates or methacrylates, titanates, and compounds containing a chelating ligand, such as phosphine, mercaptan, and acetoacetate. When present, coupling agents typically will be in amounts up to 10 percent by weight, and preferably in amounts of 0.1-3.0 percent by weight, of the maleimide and other monofunctional vinyl compound.
In addition, the encapsulant compositions may contain compounds that lend additional flexibility and toughness to the resultant cured encapsulant. Such compounds may be any thermoset or thermoplastic material having a Tg of 50xc2x0 C. or less, and typically will be a polymeric material characterized by free rotation about the chemical bonds, such as can be obtained by the presence of carbon-carbon double bonds adjacent to carbon-carbon single bonds, the presence of ester and ether groups, and the absence of ring structures. Suitable such modifiers include polyacrylates, poly(butadiene), polyTHF ( polymerized tetrahydrofuran), CTBN (carboxy-terminated butyronitrile) rubber, and polypropylene glycol. When present, toughening compounds may be in an amount up to about 15 percent by weight of the maleimide and other monofunctional vinyl compound.
If siloxane moieties are not part of the maleimide or vinyl compound structure, siloxanes can be added to the package encapsulant formulations to impart elastomeric properties. Suitable siloxanes are the methacryloxypropyl-terminated polydimethyl siloxanes, and the aminopropyl-terminated polydimethylsiloxanes, available from United Chemical Technologies.
Other additives, such as adhesion promoters, may also be added as needed. The kinds and amounts of adhesion promoters that may be used are known to those skilled in the art.
Another embodiment of this invention includes the maleimides having the formulae [Mxe2x80x94Ar m]nxe2x80x94Q and [Mxe2x80x94Z m]nxe2x80x94Ar as described herein in which Q and Z can be an ester having the structure 
or the structure 
in which
p is 1 to 100,
each R3 can independently be an alkyl or alkyloxy chain having up to 100 atoms in the chain, which chain may contain aryl substituents, or
a siloxane having the structure xe2x80x94(CR12)exe2x80x94[SiR42xe2x80x94O]fxe2x80x94SiR42xe2x80x94(CR12)gxe2x80x94 in which the R1 substituent independently for each position is H or an alkyl group having 1 to 5 carbon atoms, the R4 substituent independently for each position is an alkyl group having 1 to 5 carbon atoms or an aryl group, e and g are independently 1 to 10 and f is 1 to 50.
Another embodiment of this invention includes the vinyl compounds having the structures 
as described herein in which B is C, S, N, O, C(O), C(O)NH or C(O)N(R8), in which R8 is C1 to C5 alkyl.
Another embodiment of this invention includes the vinyl compounds having the structures 
as described herein in which Q and Z can be an ester having the structure 
or the structure 
in which
p is 1 to 100,
each R3 can independently be an alkyl or alkyloxy chain having up to 100 atoms in the chain, which chain may contain aryl substituents,
or a siloxane having the structure xe2x80x94(CR12)exe2x80x94[SiR42xe2x80x94O]fxe2x80x94SiR42xe2x80x94(CR12)gxe2x80x94 in which the R1 substituent independently for each position is H or an alkyl group having 1 to 5 carbon atoms, the R4 substituent independently for each position is an alkyl group having 1 to 5 carbon atoms or an aryl group, e and g are independently 1 to 10, and f is 1 to 50.
Another embodiment of this invention includes the composition as described herein containing an anionic or cationic curing initiator. The types and useful amounts of such initiators are well known in the art.